PHYSICS:
Conjuring Matter From Light

David Ehrenstein

Turning matter into light, heat, and other forms of energy is
nothing new, as nuclear bombs spectacularly demonstrate. Now a team of
physicists at the Stanford Linear Accelerator Center (SLAC) has demonstrated
the inverse process--what University of Rochester physicist Adrian Melissinos,
a spokesperson for the group, calls "the first creation of matter out of
light." In the 1 September Physical Review Letters, the
researchers describe how they collided large crowds of photons together so
violently that the interactions spawned particles of matter and antimatter:
electrons and positrons (antielectrons).

Physicists have long known that this kind of conjuring act is possible, but
they have never observed it directly. The experiment is also a proof of
principle for a technology, based on intense laser beams boosted to enormous
energies with the help of SLAC's electron beam, for exploring a theory known
as quantum electrodynamics. QED describes electromagnetic fields, such as
those of light, and their interactions with matter, and its predictions are
notoriously accurate. But physicists are eager to study it at so-called
"critical" electromagnetic fields--fields so strong that their
energy can be converted directly into the creation of electrons and positrons.

To create a field as close as possible to critical, the 20-physicist
collaboration started with a short-pulse glass laser that packs a
half-trillion watts of power into a beam measuring just 6 micrometers across
at its narrowest point, resulting in extraordinary intensities. To increase
the energy of the photons, the team collided the pulses with SLAC's
30-micrometer-wide pulsed beam of high-energy electrons--a feat that required
precise alignment and synchronization. When laser photons collided head-on
with the electrons, they got a huge energy boost, much like ping-pong balls
hitting a speeding Mack truck, changing them from visible light to very high
energy gamma rays. Because of the laser's intensity, these backscattered gamma
photons sometimes encountered several incoming laser photons simultaneously; a
collision with four of them concentrated enough energy in one place to produce
electron-positron pairs.

Melissinos views the result as the first direct demonstration of
"sparking the vacuum," a long-predicted phenomenon. In it, the
energy of a very strong electromagnetic field promotes some of the fleeting,
"virtual" particles that inhabit the vacuum, according to QED, to
become pairs of real particles.

Electron-positron pairs are often spawned in accelerator experiments that
collide other particles at high energies, and photons produced in the
collision are what actually generate the pairs. But at least one of the
photons involved is virtual--produced only for a brief moment in the strong
electric field near a charged particle. The SLAC experiment marks the first
time matter has been created entirely from ordinary photons.

Princeton University physicist Kirk McDonald, another spokesperson for the
collaboration, which also includes the University of Tennessee and SLAC,
thinks the high-field experiments could shed light on phenomena at the surface
of a neutron star, where magnetic fields are very strong, and in other exotic astrophysical settings. On a more practical level, the conversion of light into matter could also give particle physicists a new source of positrons that are exceptionally uniform in energy and momentum.

The result is also the first step toward using powerful lasers and electron
beams to test high-field QED predictions, such as what McDonald calls
"vacuum optics"--the behavior of light in a strong-field environment. "We're exploring new regimes and trying to map out the basic phenomena," he says. Physicist Tom Erber of the Illinois Institute of Technology looks forward to the results: "Hopefully, this will open the door to future experiments which will approach [more probing] tests of QED."